1. Field of the Invention
The invention relates to a method of manufacturing a permanent magnet that is inserted into a slot of a rotor for a permanent magnet embedded motor, a permanent magnet manufactured according to that method, a rotor provided with that permanent magnet, and an IPM motor provided with that rotor.
2. Description of the Related Art
Among the various types of known motors, including brushless DC motors, is a motor that has a permanent magnet embedded rotor in which a plurality of permanent magnets are embedded in a rotor core (this type of motor is known as an interior permanent magnet (IPM) motor and will hereinafter simply be referred to as an “IPM motor”). IPM motors are used as motors in hybrid vehicles, for example.
In a motor, a coil is formed by a winding being wound in either a concentrated or a distributed manner around stator teeth. Magnetic flux is then generated by applying current to the coil, and magnetic torque and reluctance torque are generated between that magnetic flux and the magnetic flux from a permanent magnet. A coil having a distributed winding coil has a larger number of magnetic poles than a concentrated winding coil does so the magnetic flux that enters the permanent magnet of the rotor from the teeth side (or the change in that magnetic flux) is relatively continuous when the rotor is rotating. Therefore, the change in the magnetic flux density when the rotor is rotating is relatively small. In contrast, with a concentrated winding coil, the change in the magnetic flux density is relatively large so an eddy current tends to be generated in the permanent magnet, causing the permanent magnet to generate heat. This may lead to irreversible thermal demagnetization which results in a decline in the magnetic property of the permanent magnet itself.
In terms of driving motors used in recent hybrid vehicles and electric vehicles, attempts are being made, for example, to increase the rotation speed or the pole number in order to meet the demand for better motor output performance. However, increasing the rotation speed or the like increases the variation in the magnetic field that acts on the magnet, and as a result, the eddy current tends to be generated. The thermal demagnetization of the magnet brought about by the generated heat conversely lowers motor performance and reduces motor durability.
Japanese Patent Application Publication No. 2005-198365 (JP-A-2005-198365), Japanese Patent Application Publication No. 2004-96868 (JP-A-2004-96868), and Japanese Patent Application Publication No. 2006-238565 (JP-A-2006-238565), for example, attempt to prevent the eddy-current from being generated, and thus prevent the thermal demagnetization that it causes, by forming the permanent magnet from a plurality of separate pieces which are then inserted together into rotor slots.
Making the permanent magnet from a plurality of separate pieces, as described in JP-A-2005-198365, JP-A-2004-96868, and JP-A-2006-238565, for example, is an effective way to suppress the generation of an eddy current which can be generated in the permanent magnet. The separate pieces that together form the permanent magnet described in JP-A-2005-198365, JP-A-2004-96868, and JP-A-2006-238565 are formed in one of two ways, i.e., i) each of the separate pieces is manufactured separately, or ii) a permanent magnet formed to the size and shape of the inside of the rotor slot into which the permanent magnet is to be inserted is machined (i.e., cut) into a plurality of separate pieces. The latter method of machining is typically used in view of manufacturing efficiency and manufacturing cost.
The machining described above requires an expensive cutting tool that has diamond chips adhered to the outer peripheral side of a cemented carbide disk, for example. Furthermore, this cutting tool will wear down and therefore must be replaced periodically, the frequency of which increases with the number of cuts (i.e., as the number of separate pieces into which the permanent magnet is to be cut increases). As a result of these and other factors, maintenance and rising manufacturing costs with this kind of machining are major concerns.
There are also other problems with cutting the permanent magnet by machining. For example, a ferrite magnet or a rare-earth magnet such as a neodymium magnet which is a permanent magnet has a metal structure formed of main phases S that contribute to magnetism and a grain boundary phase R that contributes to coercive force, as shown in FIG. 9 which is an enlarged view of the structure of the magnet. When the permanent magnet is divided by machining, separate pieces are formed along the cut line indicated by line L1 in the drawing. As is evident from the drawing, the line L1 is formed cutting, i.e., dividing, the main phases S so the main phases S that are cut are smaller than they are prior to being cut. As a result, the residual magnetic flux density (Br) ends up being lower after the cut.
Furthermore, the grain boundary phase R expresses the coercive force with respect to the main phases S that it surrounds. However, because the covering of the grain boundary phase R which surrounds the main phases S that contact the cut surface is broken thereby exposing the main phases S, magnetic reversal tends to easily occur in the external magnetic field. It is this magnetic reversal that leads to a decrease in the coercive force of the entire magnet.